I get an error with a object when I try to make it transparent
I'm not quite sure whats happening here, but i might know a work around. Instead of making it transparent you could just stop it from rendering. You do this by hovering over the render button and pressing I. 



This will place a key frame (it should light up when one is placed). Now just click it while on the keyframe you placed and place another key frame where you want it to render.



Hope this helps